Plastic is usually made of cellulose, carbon, petroleum, or natural gas. It consists of long chain made up of many repeating molecular units. For the natural environment, plastic is a foreign body and does not biodegrade.
Pieces smaller than 5mm in size are called microplastic. Sources for microplastics in the ocean include cosmetic products, textiles such as fleece jackets, rubbish washed from land and ships that dump their plastic waste in the ocean (even though it is prohibited).
The fishing industry accounts for 10% of marine debris. Nets and fishing gear get lost or are thrown away into the ocean. These "ghost nets" continue trapping fish for many decades.
Plastic can transport plant and animal species across great distances to other regions. These passengers unsettle the balance of the sensitive ecosystems of their destinations. Plastic can also cover coral, marine sponges and mussel beds, preventing species from populating them and cutting of marine organisms from the exchange of oxygen.
Toxins end up on the perpetrators plate
Plastics often contain additives that lend the product desirable properties - but can damage animals and humans. Bisphenol A, phthalates and brominated flame retardants can adversely affect sexual development, damage genetic material or cause cancer. Pesticides and other toxins that are release into the ocean area are also absorbed by the plastic.
All of these toxins penetrate the fatty tissue of marine organisms and end up in the food chain. Particularly at risk are all those animals at the end of the food chain: sea birds, seals, whales or sharks - and not least of all, humans.